Effect of Counterions on the Adsorption of Ionic Surfactants at Fluid−Fluid Interfaces

Abstract

The equilibrium surface tension of anionic surfactant n-decyl sulfate (DS-) for various monovalent (alkali) counterions was investigated. It was found that surface activity of surface chemically pure DS- significantly increases with decreasing hydrated size of the counterion. We describe our experimental results in terms of the previously developed adsorption model, which assumes that the counterions may penetrate the Stern layer where the surfactant headgroups are adsorbed. The headgroups and counterions have a finite size that leads to the surface exclusion effects in the adsorption isotherm. The model is improved by explicitly taking into account the electric interactions between adsorbed ions in the adsorbed layer. We obtain a good correlation between the relative counterion size in the Stern layer, the measure of the area excluded by the adsorbed counterions, and the effective radius of the hydrated counterion in the solution. The limiting areas per molecule at the critical micelle concentration for the adsorbed decyl sulfate for various counterions are in good agreement with those measured by neutron scattering.

Adsorption and structure of the adsorbed layer of ionic surfactants

“…Warszynski et al [15] accounted for the adsorption of the counterions, for the difference in sizes between the surfactant ions and the counterions, for the Stern layer and for the electrostatic interaction between the adsorbed ions. Fainerman and coworkers [16,17] as well as Liggieri et al [18] took another direction -they extended the theory of Lucassen-Reynders by including reorientation effect and surface aggregation.…”

Molecular structure of ionic surfactant solution surface and effects of counter-ion therein—a joint investigation by simulation and experiment

“…Generally, the experimental methods can further be divided into macroscopic and microscopic items. The former are able to measure the macroscopic properties of the surface, for example, surface tension [1][2][3][4][5][6][7] and surface potential [3,8,9], to derive the information about the surface structure such as molecular orientation and spatial distributions of the surfactants in the surface layer. The latter are capable of directly determining the surface structure at molecular level [10][11][12][13].…”

Interfacial effects and emulsion stabilization by in situ surfactant generation through the saponification of esters

“…Thus it is safe to conclude that shielding of electrostatic repulsions increased surfactant generation rates. Though the effect of surfactant counter ion is not specifically explored here, significant research exists which addresses the effect of surfactant counter ion in general[ 40,41,42] and in in situ systems specifically[ 9,10,43]. It is generally accepted that for anionic surfactants, counter ions in the electric double layer with smaller hydrated radii allow for a more densely packed adsorption layer, resulting in a greater reduction in surface tensions and more stable emulsions.…”